(a) Field of the Invention
The present invention relates to a burst-mode optical receiver and a timing control method.
(b) Description of the Related Art
In a communication environment of a point-to-multipoint (P2MP) method such as a passive optical network, one optical line termination (OLT) unit is basically connected to an optical terminal apparatus on the subscriber's side which is represented by an optical network unit (ONU) or an optical network terminal (ONT) through a splitter. In such a communication method, a downstream from the OLT is broadcasted to all the ONTs, whereas upstreams from the respective ONTs must be received by the OLT in a time division multiplexing (TDM) method of a burst packet.
The above upstream transmission method is also called a time division multiplexing access (TDMA) method, and it requires an appropriate burst-mode optical receiver for the purpose of communication. The burst-mode optical receiver is required to have a fast response characteristic and a wide dynamic range, and to maintain high reception sensitivity for a variety of input intensities which vary according to each packet.
Passive optical networks (PON) techniques of a TDMA method which are now commercially available include an Ethernet PON (EPON) technique and a gigabit capable passive optical network (G-PON) technique. The standard for EPON provides a laser on/off function in order to avoid collision between packets. To this end, the standard defines the time domain having a maximum of 400 ns.
The standard for GPON provides higher transmission efficiency than EPON and so permits a guard time domain up to a minimum of 25.7 ns between burst packets. Further, the standard for GPON defines a preamble time of 44 bits (35.4 ns) on the basis of 1.25 Gbit/s, which is the time in which a receiver output can be stabilized and phase/frequency synchronization can be stabilized (i.e., a CDR lock).
Because of a difference in the above standard requirements, fast reset and fast stabilization response characteristics are required for every burst packet signal. To this end, a media access controller (MAC) for GPON controls the burst packets according to a series of standard protocol methods. Further, the MAC generates a reset signal between the burst packets and provides means for allowing the burst-mode optical receiver to be prepared to reset a state determined for a previous packet signal and to receive a next packet signal.
Signals transmitted by the transmitter of an ONT/ONU are inputted to the photodiode (PD) of an OLT receiver as digital optical signals of a burst packet form for TDMA. Further, current signals of an analog form, photo-electrically converted by the PD as in a general optical receiver, are converted into voltage signals by a trans-impedance amplifier (TIA), converted into differential signals that are insensitive to noise, amplified as digital electric signals having an appropriate voltage level whose logic 1 and 0 can be determined through a limiting amplifier (LA), and then stabilized and output.
In a series of the conversion processes, the burst-mode optical receiver for PON has to perform fast signal processing and stable level conversion for each of the burst packet signals having a different input intensity without the distortion of the signal, while fulfilling a timing condition required for the PON. Such an operating requirement characteristic is not simple in the burst-mode optical receiver for GPON which has to satisfy a short guard time and a preamble time.
That is, the burst-mode optical receiver for GPON requires a wide dynamic range and a fast response characteristic at the early stage of a burst and must use a step AGC method, together with a TIA gain digitalized according to the input intensity, rather than a consecutive automatic gain control (AGC) method which is chiefly used. Further, the burst-mode optical receiver for GPON must include an automatic offset control (AOC) function for a burst packet differential signal in order to efficiently control each circuit in synchronism with the input timing of an external reset signal and each burst packet.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.